In the chemical and petrochemical industry there is a wide variety of reactions wherein a liquid reaction mixture is contacted with a catalyst that comprises silica and/or a silicate. The liquid reaction mixture may comprise a liquid solvent including water, but also one or more liquid reactants. Many chemical reactions are conducted in the presence of a catalyst. Especially when the catalyst is solid, catalysts that comprises silica and/or silicates are widely used. Examples include the production of heterocyclic nitrogen compounds including piperazine and carbon-substituted alkyl derivatives, cycloaliphatic amines from cycloalkanols and morpholine and carbon-substituted alkyl derivatives (cf. U.S. Pat. No. 3,152,998) and the conversion of furan to 1,4-butanediol and tetrahydrofuran in a dicarboxylic acid and water in the presence of a catalyst of nickel/copper/chromium on a support (cf. U.S. Pat. No. 4,146,741).
Zeolites, especially aluminosilicate zeolites, are also well-known for their catalytic activity. In many chemical reactions they are being used for such reasons. In the petrochemical industry they find use as catalysts in a variety of treating reactions, including hydrocracking, hydrotreating, reforming, dewaxing and catalytic cracking of crude oil and refinery streams. Zeolites may also be used in a variety of chemical reactions, including hydration and dehydration of various chemicals, additions to and eliminations from alcohols and acids such as etherifications and esterifications, hydroformylations, oxidations, aldol condensations and additions to epoxides.
Silica may be used in a catalyst as a carrier of the catalytically active material, such as a metal compound. It may also be used as the catalytic material itself, e.g. in the form of silica-alumina, or as a binder for catalytically active material, such as a zeolite. In R. M. Ravenelle et al., J. Phys. Chem. C, 2010, 114, 19582-19595, a study has been described wherein zeolite Y and ZSM-5 were treated in liquid water at 150 to 200° C. under autogenic pressure. It was noted that zeolite Y was transformed into amorphous material. The authors do not understand the mechanism although it is suggested that the hydrolysis of siloxane bonds plays a role. They conclude that the stability of zeolite catalysts in high temperature aqueous environments needs to be considered carefully. In H. van Bekkum et al., Introduction to Zeolite Science and Practice, Elsevier, page 828, it is observed that some high silica zeolites are stable in aqueous acid medium at moderate temperatures, but that generally low silica zeolites (A, X, Y) are not stable under conditions of low pH, and care should be taken when such zeolites are used in a low pH slurry technique as catalysts. It is therefore an objective of the present inventors to find a solution to the issue around the stability of silica or silicate-containing catalysts with the concurrent loss of catalytic activity. Since, although some catalysts that contain silica and/or silicates tend to have relatively stable structures, it has been observed that in use a loss of catalyst may occur. Generally, this catalyst loss is due to attrition, such as in the case of catalytic cracking wherein a fluid stream of reactant and catalyst particles comprising zeolites and binder material, is passed through reactors causing contact between the particles which leads to attrition, to the formation of catalyst fines and ultimately to catalyst losses. Presently, it has been found that the reduction in catalytic activity when silica- or silicate-containing catalysts are used in an aqueous reaction mixture, is caused by solubilisation issues. It appeared that under certain circumstances silicate material from the catalyst is dissolved, which leads to a loss of catalyst material, and hence to a reduction in catalytic activity.
In WO 99/42214 catalyst supports that are soluble in acid or neutral aqueous solutions are treated with a modifying compound. Such a compound may comprise silicon. In such cases the support is treated with a silicon precursor by which the silicon precursor is introduced into or onto the support via impregnation, subsequent drying and calcination, so that silicon remains. The support may be alumina, titania or magnesia. Hence, this method is unsuitable for silica and/or silicates-containing catalysts.
Whereas the catalyst losses for silica and/or silicates-containing catalysts so far have been dealt with by the addition of extra fresh catalyst, it has now been found that the catalyst loss, and hence a reduction in catalytic activity can be counteracted when a silicon compound is added to the reaction mixture in question. Accordingly, the present invention provides a process for carrying out a reaction wherein a liquid reaction mixture is contacted with a catalyst that comprises silica and/or a silicate, wherein a silicon compound that is soluble in the liquid reaction mixture is added to said reaction mixture before being contacted with the catalyst.
It appeared that when a soluble silicon compound was added to the liquid reaction mixture, the dissolution problems did not occur and catalyst losses were prevented. Without wishing to be bound by any theory, it is believed that the addition of the silicon compound results in a liquid mixture wherein the solubility of the silica and/or silicate-material is limited such that any further dissolution of solid material from the catalyst is suppressed or completely prevented. It is believed that when the liquid mixture constitutes a saturated solution of the silicate material the absolute dissolution may be completely avoided.